Soft switching technique has been used in power converters to reduce switching losses and alleviate electromagnetic interference (EMI). At present, there are two main topologies of soft switching inverters, resonant dc link and resonant snubber. The active clamped resonant dc link (ACRDCL) converter in U.S. Pat. No. 4,864,483 of Divan et al, issued Sep. 5, 1989, the auxiliary quasi-resonant dc link (AQRDCL) converter in U.S. Pat. No. 5,172,309 of De Donker, issued Dec. 15, 1992, and the voltage clamped parallel resonant (VCPR) converter in U.S. Pat. No. 5,559,685 of Lauw, issued Sep. 24, 1996, are examples of resonant dc link inverters. They are hereby incorporated by reference. Auxiliary resonant snubber inverters (in other words, the auxiliary resonant commutated pole (ARCP) or resonant snubber inverters (RSI)), represented in U.S. Pat. No. 5,047,913 of De Donker et al, issued Sep. 10, 1991, U.S. Pat. No. 5,710,698 of Lai et al, issued Jan. 20, 1998, U.S. Pat. No. 5,572,418 of Kimura et al, issued Nov. 5, 1996, and U.S. Pat. No. 5,574,636 of Lee et al, issued Nov. 12, 1996, belong to the second category, resonant snubber inverters.
The resonant snubber inverters (FIGS. 1–4) employ two resonant capacitors and one resonant inductor for each phase leg to achieve soft switching. In spite of their advantages of lower EMI, dv/dt, and switching losses, these soft switching inverters have common problems compared to the resonant link inverters and traditional hard-switching inverters: (1) excessive number of additional active and passive components; (2) high current stress on the main switching devices, and (3) poor reliability. Accordingly, soft switching technology is expected to be used for medium or high power (>100 kW) applications and special load/environment requirements, such as EMI-sensitive equipment, etc.
The resonant link inverters have advantages over the resonant snubber inverters in terms of less component count and low cost. In the ACRDCL converter (FIG. 5), a resonant circuit, incorporated with an active clamping switch and clamping capacitor, is used as an interface between a dc power supply and the dc bus of an inverter. The ACRDCL resonates periodically, bringing the dc bus voltage to zero once each resonant cycle. The inverter switching devices are switched on and off at zero voltage instants of the resonant dc link, thus achieving lossless switching. However, the ACRDCL converter has some disadvantages, such as, high voltage stress across the inverter switches and continuous resonant operation of the dc link. To overcome the disadvantages of the ACRDCL converter, the auxiliary quasi-resonant dc link (AQRDCL, FIG. 6) converter has been developed. The AQRDCL converter is employed to achieve soft-switching in an inverter coupled to a dc power supply via a resonant dc link circuit. The resonant dc link circuit includes a clamping switch limiting the dc bus voltage across the inverter to the positive rail voltage of the dc supply and auxiliary switching device(s) assisting resonant operation of the resonant bus to zero voltage in order to provide a zero-voltage switching opportunity for the inverter switching devices as the inverter changes state. The AQRDCT converter embraces its own problems such as high current stress because the clamping switch Sc and diode Dc (FIG. 6) have to carry the full dc current, although it does not have the high voltage stress problem of the ACRDCL. The VCPR converter (FIG. 7) was developed to reduce the current stress of the link switches (Sc1 and Sc2). However, the dc current that delivers dc power to the inverter has to still flow partly through the switches and partially through the resonant inductor (LR).
Despite their advantages and advances, the ACRDCL converter, the AQRDCL converter, and the VCPR converter have the following common disadvantages: (1) The resonant dc link circuit acts as an interface (i.e., a dc-to-dc converter) between the dc power supply and the inverter and needs to transmit power and to carry dc current from the dc power supply to the inverter or from the inverter back to the dc power supply via switch(es) and/or resonant component(s), which can lead to significant power losses; (2) The voltage clamping, voltage control, and charge balancing become difficult due to the dc power transmission; (3) The current stress on the auxiliary switch(es), clamping switch(es), and resonant inductor is high (at least as high as that on the inverter main switches); and (4) Two resonant dc link circuits are needed for an ac-to-dc-to-ac converter/inverter system to implement soft-switching at both ac-to-dc power conversion stage and dc-to-ac power conversion stage.